Microfiber cloth and Starsan is as rough as I have ever been. I haven't a single scratch in mine after 3 years because...
1. I never let anything dry on my fermenter as I clean them immediately
2. My kitchen faucet has good pressure and the spray tip is like a laser
3. I never use anything rougher than my calloused fingerprints as an abrasive.
4. My fermenters always or full of Wort, Beer, or StarSan.
3 or 4 times I have used scent free Oxiclean to get troubling krausen rings off